The present invention generally relates to an ignition timing control system for an automobile internal combustion engine and, more particularly to an ignition timing control system of a type operable to retard the ignition timing for a predetermined time when knocking occurs in the engine.
As is well known to those skilled in the art, the ignition timing at which a spark plug is set to fire is determined in consideration of the engine operating condition so that the automobile engine can work to its optimum efficiency. For this purpose, an ignition timing control system has long been employed to control the ignition timing. However, the prior art ignition timing control system includes detectors for detecting the engine speed (i.e. the number of revolutions of the engine) and the negative pressure inside the engine fuel intake system, respectively, and is so designed that, when the engine speed so detected is high or low, the ignition timing is advanced or retarded, respectively, and also that, when the load imposed on the engine is small or large, the ignition timing is advanced or retarded, respectively, whereby the ignition timing is maintained so as to achieve the MBT (Minimum Advance for Best Torque) value. An example of this prior art ignition timing control system is disclosed in Japanese Patent Laid-open Publication No. 53-129740, laid open to public inspection on Nov. 13, 1978.
However, it has been found that, when the ignition timing is selected attaching importance to the engine torque, the engine tends to knock before the ignition timing attains the MBT, particularly when the engine is operated under low speed, high load conditions. This in turn results in loss of the safe operability of the engine.
In order to obviate the above described problem, the ignition timing control system is additionally provided with a knocking detecting device operable to cause the ignition timing control system to retard the ignition timing for a predetermined period of time in response to an output signal generated from the knocking detecting device when the engine knocks. An example of the prior art ignition timing control system utilizing the knocking detecting system is disclosed in, for example, Japanese Patent Laid-open Publication No. 54-116525, laid open to public inspection on Sept. 10, 1979.
However, the system disclosed in the second mentioned publication is susceptible to a surging phenomenon in which the advance and retard of the ignition timing are repeated in a short period of time before and after the occurrence of the engine knocking.
However, according to, for example, Japanese Patent Laid-open Publication No. 54-153928, laid open to public inspection on Dec. 4, 1979, there is disclosed an ignition timing control system effective to substantially eliminate the surging phenomenon. However, since the ingition timing control system disclosed in this publication is so designed as to retard the ignition timing by the detection of the occurrence of the engine knocking and to maintain the retarded ignition timing for a predetermined period of time subsequent to the occurrence of the engine knocking for the purpose of avoiding the surging phenomenon, the ignition timing tends to remain retarded even when and after the engine which has been operated under the low speed, high load operating condition has subsequently become operated under any operating condition other than the low speed, high load operating condition. Accordingly, with the system disclosed in the last-mentioned publication, there is a possibility that the spark plug is set to fire at an improper ignition timing even though the engine is operated under the operating condition wherein no tendency of engine knocking occurs. This in turn results in not only a loss of engine operating efficiency but also in increased fuel consumption.